1. Field of the Invention
The present invention generally relates to wireless communications, and more particularly, to a communication node and a communication method using a multihop scheme and a multiple-input multiple-output (MIMO) scheme.
2. Description of the Related Art
In recent years and continuing, a system based on a combination of a multihop scheme and a MIMO (or multi-antenna) scheme, which system is referred to as a MIMO multihop system, is getting attention. In a multihop scheme, signals are transmitted from a source node to a destination node (or a target node) via one or more relay nodes located between the source and the destination. This system has advantages of expanded coverage by relaying signals, theoretically un-limited signal transmittable areas, and quick establishment of a wireless network. With a MIMO system, multiple transmission antennas and multiple receiving antennas are used to transmit and receive signals in order to increase communication capacity through efficient use of space.
Signal transmission is performed in a MIMO multihop system in the following steps. First, a signal S transmitted from a source node is received at a relay node. The received signal Y at the relay node is expressed asY=HS+n  (1)where H denotes a channel matrix between the source and the relay node, S denotes a transmission signal vector, and n denotes noise. Then, the transmission signal S is detected by a zero-forcing (ZF) method. This method is to detect the transmission signal S by calculating a pseudo inverse matrix W1=(HHH)−1HH, and multiplying the received signal by the pseudo inverse matrix W1, together with a normalization coefficient. This process is expressedW1Y=S+W1n.  (2)The superscript H in the Pseudo inverse matrix W1 denotes a conjugate transpose.
Norm for an arbitrary matrix A is defined by∥A∥=(Tr(E[AAH]))1/2  (3)where symbol ∥•∥ represents norm, symbol Tr(•) represents the total sum of the diagonal elements of the matrix in the parenthesis, that is, a trace, and symbol E[•] represents averaging the quantities in the bracket. In particular, norm ∥V∥ for vector quantity V=(v1, v2, . . . , vM)T is expressed as∥V∥=[|v1|2+|v2|2+ . . . +|vM|2]1/2  (3)′where superscript T represents transpose. The above-described pseudo inverse matrix corresponds to a Moore-Penrose inverse matrix. In general, the Moore-Penrose inverse matrix B is defined as a m×n matrix that establish BA=I for a n×m matrix A. In the illustrated example, W1H=I holds with respect to matrix H.
Then, pseudo inverse matrix W2=(GHG)−1GH is calculated, where G denotes a channel matrix between a relay node and the destination node. Both sides of Equation (2) are multiplied by this pseudo inverse matrix W2 and a normalization coefficient E. This relation is expressed asE(W2W1)Y=EW2(S+W1n)  (4)where E=1/(∥W1∥ ∥W2∥)*(Ps/(Ps+σn2)1/2 holds, Ps denotes transmit power, and σ2 is variance of noise.
The thus calculated signal is transmitted from a relay node to the destination node. The signal YR received at the destination node is expressed asYR=GEW2W1Y+nR  (5)where nR denotes a noise component. Equation (5) can be rewritten asYR=E(S+W1n)+nR  (6)based on the definitions of W1 and W2.
In this manner, the transmission signal S can be acquired promptly at the destination node. Such a MIMO multihop system is described in, for example, Rohit U. Nabar, et al., “Capacity Scaling Laws in MIMO Wireless networks”, Allerton Conference on Communication, Control, and Computing, Monticello, Ill., pp. 378-389, October 2003.
From Equation (6), it is understood that the received signal YR contains a factor 1/(∥W1∥ ∥W2∥) with respect to the transmission signal S. Such factors ∥W1∥ and ∥W2∥ are indispensable for transmit power control performed at the relay node. However, since W1 and W2 are inverse matrices of channel matrices H and G, respectively, which are subjected to influence of noise amplitude, signal quality is inevitably degraded. In addition, Equation (6) contains the noise component “n”, which is introduced during propagation from the source to the relay node in such a manner that greatly affects the received signal. Accordingly, as the number of hops increases, signal degradation due to the noise become conspicuous.
In addition, consideration has to be made of a wireless communication system in which signals are relayed simultaneously from multiple source nodes to associated destination nodes via relay nodes. In such a system, the signal received at the destination node contains not only influence of the desired source node, but also that of the other source nodes. There is concern in such a system that the noise is amplified at the relay node, and that the received signal quality at the destination node is particularly degraded.